This invention relates to an optical recording medium having a phase change layer and a method for recording information in such medium.
Highlight is recently focused on optical recording media capable of recording information at a high density. Typical optical recording mediums are write once media which can be recorded only once and which can not be rewritten, and rewritable media wherein repeated rewriting has been enabled. Improvement in the recording density and increase in the data transmission rate are always required for an optical recording medium.
Of the rewritable recording media, those of phase change type are recorded by changing the crystalline state of the recording layer by irradiating a laser beam, and read by detecting the change induced in the recording layer by such change in reflectivity in the crystalline state.
In the phase change medium which can be rewritten by overwriting, amorphous record marks are formed by irradiating the medium with a laser beam of recording power level to melt the crystalline recording layer and quenching the molten recording layer to thereby form the amorphous record marks. In the erasure, the medium is irradiated with a laser beam of erasing power level to heat the recording layer to a temperature of not less than the crystallization temperature and less than the melting temperature followed by gradual cooling to thereby crystallize the amorphous record marks. Accordingly, the overwriting can be accomplished by irradiating a single laser beam with its intensity modulated. In the recording of such phase change medium at a high speed, the rate determining factor is crystallization speed of the recording layer, namely, the transformation speed from the amorphous to the crystalline state. The change from the crystalline to the amorphous states can be accomplished within the period of several nano seconds while crystallization from the amorphous to the crystalline state requires maintenance at a temperature at or above the crystalline temperature for at least a predetermined period.
An as-deposited phase change layer is generally amorphous. In the meanwhile, record marks formed by melting and quenching the crystalline recording layer are also amorphous. The as-deposited phase change layer and the amorphous record marks share the common feature that they are amorphous. The amorphous state of the as-deposited amorphous recording layer, however, is more stable than the amorphous record marks, and even when the as-deposited phase change medium were overwritten as described above, crystallization of the region which had been irradiated with the laser beam of erasing power level is difficult. Accordingly, there is a need to complete the initialization of the recording layer (initialization of the entire surface) before the overwriting operation. If the initialization is difficult, production cost will be increased since the initialization should be conducted by using a laser beam of higher power and at a lower speed. Also known is a write once medium wherein crystalline record marks are formed in the as-deposited recording layer, namely, in the amorphous recording layer. The crystallization of the as-deposited recording layer is quite difficult as described above, and there is a high demand for a means capable of readily crystallizing the as-deposited amorphous recording layer. In addition, if amorphous record marks can be crystallized (erased) at a higher speed, data transfer rate in the overwriting operation can be increased.
Various proposals have been made to facilitate crystallization of the as-deposited amorphous recording layer or to speed up the erasure of the record marks. Proposals include provision of a layer in contact with the recording layer for promoting the crystallization of the recording layer, and constitution of the recording layer from a laminate of layers.
For example, JP-A 92937/1989 discloses an optical recording medium comprising a recording layer containing Te or Se as its main component and a crystal nucleus-forming layer in contact with the recording layer, wherein apparent speed of nuclei formation near the melting point has been increased. There is also disclosed that the increase in the apparent nuclei formation speed of the recording layer enables erasure of the record marks at a higher speed. In claim 4 of JP-A 92937/1989, there is described that the crystal nucleus-forming layer is amorphous immediately after the production of the optical recording medium, and once crystallized by laser beam irradiation, the layer never becomes amorphous or immediately crystallized upon irradiation with the laser beam. In other words, the stable phase for this crystal nucleus-forming layer is the crystalline phase once the layer has been crystallized even if the layer went through repeated recording and erasing operations. JP-A 92937/1989 also describes that it is preferable that the crystalline phase of the crystal nucleus-forming layer after its crystallization is the same as the crystalline phase of the recording layer. Examples of JP-A 92937/1989 disclose combination of the recording layer comprising Te57In18Au25 and the crystal nucleus-forming layer comprising Te67Au33.
WO98/47142 discloses an optical information recording medium wherein a crystallization-promoting layer is provided in contact with the recording layer comprising a Ge-Sb-Te-based alloy. This crystallization-promoting layer has a crystal structure of face centered cubic lattice which is the same as that of the recording layer, or a rhombohedral lattice which does not include Te. Initialization of the recording layer is not required in this medium since the recording layer is crystallized at the time of its formation owing to the provision of the crystallization-promoting layer and the recording layer in contact with each other. There is disclosed that the adjacent crystallization-promoting layer and recording layer turns out to be in mixed state. In Examples of WO98/47142, the recording layer comprises a composition based on Ge2Sb2Te5, and the crystallization-promoting layer contains PbTe, Bi2Te3, Sb, or Bi. In Comparative Examples, the crystallization-promoting layer contains W (body centered cubic lattice), Te (hexagonal system), Sb2TeSe2 (rhombohedral lattice), Sb2Te3 (rhombohedral lattice), or Ag2Te (monoclinic system), CrTe (hexagonal system).
JP-A 185289/1999 discloses a write once optical information-recording medium which has a phase change recording layer on at least one surface of the substrate, and a layer comprising a semiconductor material immediately on and/or under the recording layer. In this medium, when the recording layer is crystallized, the shape of the unit cell constituting the crystal face parallel to the substrate in the recording layer matches with the shape of the unit cell constituting the most dense face of the semiconductor material layer. The invention described in JP-A 185289/1999 attempts to reduce the jitter by providing such semiconductor material layer, and adequately selecting the material used for each layer so that absolute value of the lattice mismatch between the recording layer and the semiconductor material layer does not exceed 10%. JP-A 185289/1999 does not explicitly indicate the crystallization-promoting effect realized by providing the semiconductor material layer in contact with the recording layer. JP-A 185289/1999, however, describes that it has been estimated that, when the recording layer had been crystallized, deformation of the lattice that takes place at the boundary with the adjacent layer prevents crystallization, and hence, invites increase in the jitter. The compounds indicated in JP-A 185289/1999 as exemplary compounds for use in the semiconductor material layer include BaO, AgCl, BeTe, GaAs, AlAs, YSb, YP, ZnSe, ThS, SnAs, YSe, AgBr, ThP, LaS, ScSb, ThSe, CaSe, PbS, ScBi, ThAs, BiSe, InAs, YTe, GaSb, PbSe, SnSb, AlSb, CuI, SrSe, SnTe, ThSb, CaTe, BaS, LaTe, PbTe, BiTe, SrTe, AgI, InSb, CdTe, Sb2Te3, Bi2Se3, and Bi2Te3. The materials indicated for use in the recording layer include alloys containing at least one of Te, Sb and Se, among which Texe2x80x94Gexe2x80x94Sb alloys and Inxe2x80x94Sbxe2x80x94Texe2x80x94Ag alloys being indicated as the most preferable. The Inxe2x80x94Sbxe2x80x94Texe2x80x94Ag alloy used in the Examples is Ag2.6In3.7Sb64.2Te29.5. In the medium described in JP-A 185289/1999, crystalline pits (record marks) are formed in the amorphous recording layer. It should be noted that, unlike the WO98/47142, JP-A 185289/1999 does not explicitly refer to the state of the semiconductor material layer after completion of the medium. JP-A 185289/1999, however, discloses that it is not the interdiffusion between the recording layer and the compound semiconductor layer that takes place.
JP-A 226173/1998 discloses an optical recording medium which has a recording layer comprising a laminate of a Sb-based thin layer containing Sb as its main component and a reactive thin layer containing In, Ag and Te (and optional Sb) as its main components or Ge and Te (and optional Sb) as its main components, and wherein the mixing of both thin layers generates a phase change material. In this medium, the treatment of mixing both thin layers is generally conducted after forming the recording layer by continuously irradiating the layer with a laser beam. In the area where the layers have been mixed, the amorphous phase such Agxe2x80x94Sbxe2x80x94Te phase is dispersed in the Sb crystalline phase, and the reflectively is lower than that before the mixing but higher than the amorphous region (record marks). The medium is overwritten after the mixing treatment by the procedure normally used in a phase change medium. In the region which has been irradiated with the laser beam of erasing power level, crystallization into AgSbTe2 takes place to increase the reflectivity.
JP-A 73692/1999 discloses an optical recording medium which has a recording layer comprising a laminate of a Te-based thin layer containing Te as its main component and reactive thin layer containing Ge and/or Sb as its main component, and wherein the mixing of both thin layers generates a phase change material. In this medium, the treatment of mixing both thin layers is conducted after forming the recording layer by continuously irradiating the layer with a laser beam. In the area where the layers have been mixed, the amorphous phase such as Gexe2x80x94Sb phase is dispersed in the Te crystalline phase, and the reflectively is lower than that before the mixing but higher than the amorphous region (record marks). The medium is overwritten after the mixing treatment by the procedure normally used in a phase change medium. In the region which has been irradiated with the laser beam of erasing power level, crystallization into GeTe2 or Sb2Te3 takes place to increase the reflectivity.
JP-A 342629/1993 discloses an information recording medium wherein a high speed initialization has been enabled by providing an easily crystallizable auxiliary layer in contact with the recording layer comprising a phase change material. In this medium, the auxiliary layer has a composition containing at least 50 atom % of Se or at least 70 atom % of Te, and average composition of the auxiliary layer and the recording layer is Ge2Sb2Te5, GeSb2Te4, or In3SbTe2. In other words, the auxiliary layer and the recording layer of this medium reacts with each other to thereby constitute a normal composition of the phase change material.
In addition to the layers as described above, it is also known to use a non-metal layer for the purpose of promoting the crystallization of the amorphous phase change layer.
For example, JP-A 149322/2000 discloses a non-initialized phase change optical recording medium comprising a phase change layer and a crystallization-inducing layer provided in contact with the phase change layer. This crystallization-inducing layer is a light-transmitting layer which is crystalline. JP-A 149322/2000 discloses that overwriting of the as-deposited recording layer is enabled by the provision of the crystallization-inducing layer. JP-A 149322/2000 also indicates that, surface of a crystalline thin film generally function as crystallization nuclei when the crystalline thin film is provided in contact with an amorphous thin film, and JP-A 149322/2000 makes use of this function. JP-A 149322/2000 indicate use of cerium oxide and zinc sulfide for the crystallization-inducing layer, and also, use of a ternary alloy comprising Ge, Sb and Te such as Ge2Sb2Te5 and a ternary alloy comprising In, Sb and Te for the recording layer.
SPIE Vol. 3401,24-32 and JP-A 195747/1994 disclose that crystallization speed of the phase change layer can be increased by providing a layer of germanium nitride or silicon nitride in contact with the phase change layer having the composition near Ge2Sb2Te5.
As described above, it has been known to provide a layer in contact with the recording layer for the purpose of promoting the crystallization of the recording layer. The inventors of the present invention, however, found that the crystallization-promoting layers used in the prior art as described above were often insufficient in promoting the crystallization, or caused excessive influence in the heat transfer in the medium to invite partial amorphizing of the region which should be crystallized. In addition, the recording layer of the prior art medium was often constituted from a plurality of layers and the initialization was promoted by mixing or reacting of such layers. In such a case, the crystallization-promoting effects were lost once the recording layer was initialized, and the erasing (recrystallization) speed of the amorphous record marks could not be increased.
The present invention has been completed in view of such situation, and an object of the present invention is to provide a medium having a phase change recording layer wherein crystallization of the recording layer is facilitated, and at the same time, wherein the region to be crystallized can be crystallized at an accurate dimension.
Such objects are attained by the present invention as described in (1) to (5), below.
(1) An optical recording medium having a recording layer comprising at least one phase change layer and at least one functional layer in contact with said phase change layer, wherein
the phase change layer is amorphous in its as-deposited state, and crystals having Fm3m structure or R3m structure are produced upon crystallization of the phase change layer, and
the functional layer is crystalline in its as-deposited state containing crystals having Fm3m structure, and the material constituting the functional layer is the one which exhibits a thermal conductivity of 0.03 to 5 W/cmK as measured in thin film form of 100 nm thick.
(2) An optical recording medium having a recording layer comprising at least one phase change layer and at least one functional layer in contact with said phase change layer, wherein
the phase change layer is amorphous in its as-deposited state, and crystals having Fm3m structure or R3m structure are produced upon crystallization of the phase change layer, and
the functional layer is crystalline in its as-deposited state containing crystals having Fm3m structure, and the functional layer contains at least one element selected from Al, Cu, Ag, Au, Ni, Pd, Pt and Rh as its main component at a content of 60 to 100 atom %, and the functional layer is free from metal elements of Groups 15(Vb) and 16(VIb).
(3) An optical recording medium according to the above (1) or (2) wherein said phase change layer has a composition in atomic ratio of:
(SbxTe1xe2x88x92x)1xe2x88x92yMyxe2x80x83xe2x80x83(I)
wherein M represents elements other than Sb and Te, and x and y satisfy the relations:
0.55xe2x89xa6xxe2x89xa60.90,
and
0xe2x89xa6yxe2x89xa60.25.
(4) An optical recording method for recording the optical recording medium of any one of the above (1) to (3), wherein crystalline record marks are formed by irradiating the amorphous phase change layer with a single laser beam whose intensity is modulated.
(5) An optical recording method for recording the optical recording medium of any one of the above (1) to (3), wherein amorphous record marks are formed by irradiating the crystalline phase change layer with a single laser beam whose intensity is modulated.